What is an appropriate fitness function for a simulated self-driving car?
You're correct that it is related to the fitness function, but only indirectly.  Recall that a function is the mathematical embodiment of a conceptual relationship.  The model is a motion model.  Proximity is not simply applying the distance formula $D = \sqrt{(x_2 - x_2)^2 + (y_2 - y_2)^2}$.

Proximity as we "feel it" when driving a car is based on the risks we see in the trajectories of ourselves and the other moving objects along with the stationary objects.  Mathematically, this is an integral over time of a distribution of probable locations.

Look for work in the aeronautics industry about collision avoidance.  The mathematics and algorithms are well developed for antiaircraft weaponry (where the fitness function is the inverse of the one you want) and air traffic control, where the fitness is like yours except with an altitude dimension.